I'd put about 60% of the blame for Dad's initial, damaging decision on this on the daughter who asked him not to talk her sister down the aisle. She was being petty here- a small private ceremony would not have been comparable to the large wedding she had had planned, and given the circumstances, she should have been big about it and not acted as if her sister was trying to "steal her thunder".

Her response to this suggests there were other issues between the sisters, that perhaps she cared more about being seen as "the winner" on this than anything else

That said, 40% of the blame here does, in my view, go to the dad- he should have realized that what she was asking was hurtful and toxic, that doing as she asked would ring a bell nothing could ever un-ring.